As shown in Figure 20.3, fill in these fields in the Table folder:
|
Name
|
STUDENT
|
|
Display Title
|
Student
|
|
Number of Rows,
Start/End
|
An estimate of the number of rows that will initially/ultimately
be stored in the table (used to estimate the size of the
initial extent for the table)
|
|
Comment
|
Contains basic information about students.
|
|
Journal
|
Checked. When used with Designer/2000, it indicates that
a journal table should be created for this table (a journal table is used to store all changes to a table, including who was responsible for the change and when it was made).
|
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Figure 20.3.
Specifying the table
characteristics.
To specify the columns in the Student table, select the tab folder labeled Column Defn.
You will see five column headings displayed: Column Name, Seq, Domain, Datatype, and
Avg Len. However, there are more fields to the right of Avg Len that can be displayed by
using the horizontal scrollbar. In this form, each row represents information about a single
column (see Figure 20.4). To add a column to the Student table, specify a value for these fields:
|
Column Name
|
The name of the column (for example, Student_ID)
|
|
Seq
|
The order in which the columns should be specified in the
CREATE TABLE statement generated by Database Designer
|
|
Domain
|
May be left blank (can be used with Designer/2000 to define
each domain and the attributes that are associated with the domain)
|
|
Datatype
|
The datatype of the column can be specified from the popup list
|
|
Avg len
|
Average length of the column (this is used for estimating the size
of the initial extent for the table)
|
If you scroll to the right, you will see some other fields displayed, including (see Figure 20.5):
|
Max Len
|
The maximum length for the column (which you may or may
not specify, depending on the datatype of the column).
|
|
Opt
|
If checked, the column is optional (the default); if not checked,the column is mandatory (NOT NULL).
|
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Figure 20.4.
Defining a column.
Figure 20.5.
Additional fields for
defining a column.
To specify a table's constraints, select the Constraints tab folder. Within the Constraints
tab folder, you will see four other tab folders that correspond to the four types of
constraints: Primary, Foreign, Unique, and Check. To specify the primary key for the Student table,
enter the constraint name, and select the Student_ID column from the poplist labeled
Column Name (see Figure 20.6).
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Figure 20.6.
Specifying the primary
key.
The Student table has another constraint: a
CHECK constraint on Year to ensure that it is
either FRESHMAN, SOPHOMORE, JUNIOR, or SENIOR. To create this constraint, select
the Validation tab folder. Select the column whose values you want to validate—in this
case, YEAR. Enter each valid value for the column, as shown in Figure 20.7.
Figure 20.7.
Specifying the
validation for a
column.
Page 543
When you are finished specifying the table's characteristics, click OK. The table will
be displayed in the diagram with the characteristics that you specified. As you can see in
Figure 20.8, the # is displayed to the left of Student_ID, signifying that the column is the
primary key. Also, all mandatory columns have a * displayed to their left.
Figure 20.8.
The created table
is displayed in a
diagram.
To follow the steps that are used to create a foreign key, let's refer to an existing diagram
that has two tables: Department and Instructor. As you can see in Figure 20.9, the Instructor
table doesn't contain Department_ID, which identifies the department with which the
instructor is associated.
To create a mandatory foreign key in the Instructor table, select the Mandatory Foreign
Key button on the tool bar (it is commonly referred to as a
crow's foot). Notice that the mouse pointer has now changed to the crow's foot. Move the crow's foot over the Instructor
table. Left-click on the Instructor table; drag the crow's foot over the Department table and
left-click. You should now notice two things (as shown in Figure 20.10):
- A mandatory foreign key labeled
DEPARTMENT_FK connects the Department and Instructor tables.
- The Department_ID has been added to the Instructor table.
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Figure 20.9.
Existing diagram
containing two tables
without a relation-
ship.
Figure 20.10.
The foreign key is
created.
Page 545
To edit the foreign key, double-click the line that defines the foreign key. A window,
labeled Edit Foreign Key, will appear (see Figure 20.11). For this foreign key, the default values
do not need to be changed.
Figure 20.11.
Editing the foreign
key.
Database Designer provides a Generation Wizard that guides you through the process
of generating a database schema from your ER diagram. To invoke the Generation Wizard,
you can select Tools | Generation Wizard from the menu. The first window displayed by
the Generation Wizard simply informs you that you can generate database objects to a file or
to a database (see Figure 20.12). Click Next to proceed to the next window.
In the next window, you must indicate where you want to create the database
objects—to a file that contains the DDL statements which will build your database or directly to
a database. By default, the DDL statements will be directed to an SQL file (see Figure
20.13). Click Next to proceed to the next window.
In the next window, you can restrict the generation of database objects to specific object
types. However, by default, all database objects will be generated (see Figure 20.14).
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